The present invention relates to an additive for lubricating oil and a lubricating oil composition containing said additive. More particularly, it pertains to an additive for lubricating oil which is suitably used for hydraulic fluid, traction drive oil, bearing oil, engine oil, etc. and a lubricating oil composition compounded with said additive. 2. Description of the Related Arts
In general, a zinc dithiophosphate (ZnDTP) which is used as an antioxidant and an anti-wear additive suffers a disadvantage that, when compounded in a lubricating oil used at a high temperature, it is highly apt to turn to a black color and further deposit-sludges.
As methods of overcoming such a disadvantage, several attempts have been made including (1) the alteration of alkyl and aryl groups in ZnDTP, (2) the alternation of alkyl and aryl species such as difference in primary, or secondary compound, difference in carbon numbers or the like, and (3) improvement in the process for producing and purifying ZnDTP, etc.
Nevertheless, the above-mentioned attempts are still incapable of suppressing the tendency of turning to a black color when the above ZnDTP is compounded in a lubricating oil used at a temperature higher than 100.degree. C., therefore, the aforementioned problem remains unsolved.
As the other method of overcoming the disadvantage, there is available a method of employing ZnDTP in combination with a detergent dispersant, thus solubilizing the decomposition product of ZnDTP. However the above-mentioned method is also incapable of suppressing the tendency of turning to a black color.
In order to solve the disadvantage of the foregoing prior art, intensive research has been made by the present inventor on the development of a novel technique capable of suppressing the tendency of turning to a black color and sludge deposition even when ZnDTP is compounded in a lubricating oil used at a high temperature.